CMOS image sensors have become increasingly prevalent in various devices, and in particular, mobile cellular telephones. A major demand within the mobile phone market is for small, thin, pocket sized devices that are still packed with various additional features, such as cameras incorporating CMOS image sensors. As a result, image sensor manufacturers are continually required to produce sensors with either a greater resolution for the same physical size, or a smaller physical size with at least the same resolution.
Resolution is determined in the first instance by the number of pixels within an image sensor, and secondly, by the resolutions of the analog to digital converters within the image sensor. That is, resolution is determined by the number of pixels available to represent the image being taken, and the number of bits required to represent the signal from each pixel, respectively.
U.S. Pat. No. 6,943,719 discloses an analog to digital converter (ADC) for a power saving image sensor. The ADC comprises a correlated double sampling (CDS) circuit and an output circuit. The CDS circuit includes an input for a voltage ramp, and an input for a signal from a pixel. Each input includes a switch to isolate the input signal from the CDS. The CDS circuit also includes first and second capacitors that are in series with the respective switches. An output of the CDS is connected to both the first and second capacitors, and is connected to the output circuit.
The output circuit comprises, in series, an inverter circuit having a bypass controlled by a switch, a capacitor, and an inverter also having a bypass controlled by a switch and a latch. The power consumption of the ADC is controlled by selectively enabling the inverter circuit, and selectively allowing the pixel signal onto one of the capacitors in the CDS circuit. The inverter is switched off when not required, and the pixel is switched off when not required. This helps to minimize or reduce power consumption of the circuit.
Prior art ADCs for image sensors have a number of drawbacks when attempting to increase resolution while also minimizing the size of the image sensor package. In particular, during correlated double sampling, kT/C noise or reset noise is cancelled by sampling the kT/C noise during reset before sampling the pixel signal onto the capacitors. The size of capacitors is determined by the kT/C noise. As the ADC circuit is required for every column in the pixel array, an increase in capacitor size can severely effect the size of the overall image sensor.